1. Field of the Invention
This invention relates to Liquid Crystal Display (LCD) and in particular to LCDs of the transflective type.
2. Description of the Related Art
LCDs can be classified according to the source of illumination. Reflective displays are illuminated by ambient light that enters the display from the front. A reflective surface, such as an aluminum or silver reflector placed in or behind the reflective display, returns light to illuminate the reflective display. Although reflective displays meet the need for low power consumption, the displays often appear rather dark and are therefore difficult to read. In addition, there are many conditions where there is insufficient ambient light, the purely reflective display is thus limited in usefulness.
In applications where the intensity of ambient light is insufficient for viewing, supplemental lighting, such as a backlight assembly, is used to illuminate the display. Although supplemental lighting can illuminate a display regardless of ambient lighting conditions, it is an expensive drain on battery life. Thus, the batteries on portable computers, for example, must typically be recharged after 2 to 4 hours of continuous backlight use. In applications where the intensity of ambient light is very strong, e.g., under an outdoor burning sun, the transmissive image illuminated only by the backlight assembly is insufficient for viewing because of poor contrast.
In an attempt to overcome the above described drawbacks of reflective and transmissive displays, some electronic displays have been designed to use ambient light when it is available and backlighting only when it is necessary. This dual function of reflection and transmission leads to the designation, xe2x80x9ctransflectivexe2x80x9d. Transflective LCDs are dual mode display devices. These devices operate either with the available ambient light in a reflective mode or with an internal backlight in a transmissive mode.
FIG. 1 illustrates a conventional transflective LCD 100 which comprises two opposing glass substrates 32 and 34 with a liquid crystal layer 36 sandwiched therebetween. Two polarizers 20 and 40 are formed on the outer surfaces of the two substrates 32 and 34. A retardation film 22 is formed between the substrate 32 and the polarizer 20, and a retardation film 24 is formed between the substrate 34 and the polarizer 40. Typically, the substrate 34 is provided with a plurality of pixel regions arranged in a matrix each with a TFT, a reflection electrode 37 (as an ambient light reflector) and a transmission electrode 38 (as a back light transmitter) which is formed at a location corresponding to the opening of the reflection electrode 37. The substrate 32 is provided with color filter elements 33 for displaying colors and a common electrode 35. While the liquid crystal layer having negative dielectric anisotropy is possible, the liquid crystal layer has the more popular positive dielectric anisotropy. Thus, when the switching element is in the xe2x80x9conxe2x80x9d state or in the xe2x80x9coffxe2x80x9d state, the light passing through the liquid crystal layer will be altered in some way, depending upon the nature of the light and the type of LCD.
In the reflective mode, the light passes through the color filter element 33 twice; however, in the transmissive mode, the light passes through the color filter element 33 only once. Therefore, each pixel region of the conventional LCD 100 is provided an extra color filter layer 33a on the color filter element 33 of the substrate 32. This arrangement can effectively prevent degraded color saturation in the transmissive mode. However, since the extra color filter layer 33a and transmission electrode 38 are formed on the two different substrates 32 and 34, the two glass substrate 32 and 34 must be precisely aligned during assembly such that the extra color filter layer 33a can be precisely formed on the region of the color filter element 33 corresponding to the transmission electrode 38. Therefore, the manufacturing process of the conventional LCD 100 may not be easily controlled and requires precise design. Accordingly, there exists a need in the art for a transflective liquid crystal display which overcomes, or at least reduces the above-mentioned problems of the prior art.
It is a primary object of the present invention to provide a transflective LCD which has good color saturation in the transmissive mode and can be easily manufactured.
The liquid crystal display (LCD) according to the present invention primarily comprises a first substrate, a second substrate and a liquid crystal layer formed between the inner surfaces of the first and the second substrates. The first substrate is provided with a plurality of pixel regions arranged in a matrix each with a switching element (e.g., TFT), a reflection electrode (as an ambient light reflector) and a transmission electrode (as a backlight transmitter). The LCDs according to the present invention are characterized by having an extra color filter layer provided in each of the pixel regions of the first substrate for improving the color saturation in the transmissive mode.
In a general aspect of the present invention, the first substrate is provided with a plurality of gate lines formed parallel to one another, and a plurality of data lines formed parallel to one another and vertically to the gate lines. The gate lines and the data lines are arranged to form a matrix of pixel regions with each of the pixel regions bounded by two adjacent gate lines and two adjacent data lines. The switching elements are formed at intersections of the gate lines and the data lines. A passivation layer is formed on the switching elements and the data lines. The passivation layer has a plurality of contact holes. An overcoat layer is formed on the passivation layer with an uneven surface closest to the liquid crystal layer (i.e., the side of the overcoat layer adjacent to the liquid crystal layer has an uneven surface) and the overcoat layer has a plurality of cavities to expose the contact holes of the passivation layer. Preferably, two polarizers are provided on the outer surfaces of the first and second substrates. A first retardation film is provided between the first substrate and the polarizer and a second retardation film is provided between the second substrate and the polarizer.
According to one embodiment of the present invention, the reflection electrode is formed on the overcoat layer such that the surface of the reflection electrode is uneven. The reflection electrode has at least one opening corresponding to the cavity of the overcoat layer, and the transmission electrode is disposed at a location corresponding to the opening of the reflection electrode. The transmission electrode and the reflection electrode are electrically connected to each other and at least one of them is electrically connected to the switching element. It is noted that the color filter layer is formed in the cavities of the overcoat layer. In this embodiment, the second substrate is provided with color filter elements for displaying colors. In the transmissive mode, light will pass through the color filter layer in the cavities and the color filter element on the second substrate, and then arrive a viewer thereby obtaining better color saturation. It will be understood that the transmission electrode is preferably disposed between the color filter layer and the liquid crystal layer. Alternatively, the transmission electrode may be also disposed under the color filter layer. In any pixel region, the color filter element of the second substrate and the corresponding color filter layer of the first substrate have the same color.
According to another embodiment of the present invention, the overcoat layer is made of a color filter material. The reflection electrode is formed on the overcoat layer such that the surface of the reflection electrode is uneven. The reflection electrode has at least one opening formed at a location outside the region corresponding to the cavity of the overcoat layer. The reflection electrode is electrically connected to the switching element through the contact hole of the passivation layer. It is noted that there is another color filter layer being formed over the reflection electrode and filling the cavity of the overcoat layer. The color filter layer has a substantially even surface closest to the liquid crystal layer, and has at least one hole. The transmission electrode is disposed on the substantially even surface of the color filter layer and electrically connected to the reflection electrode through the hole of the color filter layer. Preferably, the hole of the color filter layer is formed at a location outside the region corresponding to the cavity of the overcoat layer. In the transmissive mode, light will pass through the color filter material of the overcoat layer and another color filter layer on the reflection electrode, and then arrive a viewer, thereby obtaining better color saturation.
According to still another embodiment of the present invention, the overcoat layer further comprises a plurality of second cavities. The reflection electrode is formed on the overcoat layer such that the surface of the reflection electrode is uneven. The reflection electrode has a first opening corresponding to the cavity of the overcoat layer exposing the contact hole of the passivation layer and a second opening corresponding to the second cavity of the overcoat layer. At least a portion of the reflection electrode is electrically connected to the switching element through the contact hole of the passivation layer. It is noted that the color filter layer is formed on the reflection electrode and fills the cavities of the overcoat layer. The color filter layer has a substantially even surface closest to the liquid crystal layer, and has at least one hole. The transmission electrode is formed on the substantially even surface of the color filter layer and electrically connected to the reflection electrode through the hole of the color filter layer. Preferably, the hole of the color filter layer is formed at a location outside the region corresponding to the cavities of the overcoat layer. In the transmissive mode, light will pass through the color filter layer filling the cavities of the overcoat layer and then arrive a viewer, thereby obtaining a better color saturation.
According to still another embodiment of the present invention, the color filter layer is substituted for the overcoat layer and formed on the passivation layer. The color filter layer has an uneven surface closest to the liquid crystal layer and has a plurality of cavities to expose the contact holes of the passivation layer. The transmission electrode is provided on the color filter layer and electrically connected to the switching element through the contact hole of the passivation layer. The reflection electrode is provided on the transmission electrode such that the surface of the reflection electrode is uneven. The reflection electrode has at least one opening formed at a location outside the region corresponding to the cavity of the overcoat layer. In this embodiment, the second substrate is provided with color filter elements for displaying colors. In the transmissive mode, light will pass through the color filter layer on the passivation layer and the color filter element on the second substrate, and then arrive a viewer, thereby obtaining a better color saturation. In any pixel region, the color filter element of the second substrate and the corresponding color filter layer of the first substrate have the same color.